Any Z50-based 3-speed engine, including the numerous 72cc flavors, will bolt-in directly. K1-`81 CT70 motors are all the same, except for the SN.
You could use a K0, `82, or `91-`94. K0s have different primary gearing, so you'd need to fit a 38t sprocket at the wheel, in place of the 35t. `82 has some oddball (among CT70s) internal parts, but is virtually plug & play. `90s era lumps are 12v/CDI so you'd have some wiring work, plus battery & bulb changeouts. Any of these could work for you.